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Finally put together (need Help) Cant get it to idle properly.

Discussion in 'Norton Commando Motorcycles (Classic)' started by brian4.2, Sep 28, 2019.

  1. brian4.2

    brian4.2 VIP MEMBER

    Jun 1, 2016
    About the bike. 1972 Combat Commando.

    Crankcase oil ways have been sorted. Timing cover and oil pump upgraded by AMR.

    New main bearings., pistons and rings, cylinder head rebuilt by JimC, JS1 cam and radiused followers.

    New Amal Premiers with 220 mains, 106 and needle clips on the top grove. I have a K&N short filter because of the electric start. Chokes are present. Floats set to .080"

    The bike starts easily and pulls strongly. I can't seem to get a steady idle. I did set it up using 3/16" rods to sinc. Throttle seems fine , snaps to return as expected.

    Was a bit of blue/white smoke on the right side first few times, you can see it in the video.

    The bike surges in RPM (from 1500 to 3ooo) as I make the slightest adjustment on the idle screws. I am not really working with the mixture screws yet. I have been trying for a bit. I was hoping to run the DGR tomorrow but not running like this, its going to be crazy with 400 riders.

    Thanks for your help. Brian

    Wondering if there is anyone in the greater LA area that could have a listen and help?

    750 front Bend.jpeg 750 side Bend.jpg
  2. JimC


    Oct 12, 2007
    That head rebuild had better have been done by comnoz (Jim Comstock), JimC doesn’t know shit about building a Commando head.
  3. cliffa


    May 26, 2013
    Love your bike !

    New Premiers often contain swarf in the pilot passages unfortunately, so need stripping and blowing through before fitting.

    19 pilot jets are now recognized as a better option than the previously supplied 17’s. Is that what you have?


  4. brian4.2

    brian4.2 VIP MEMBER

    Jun 1, 2016
    Thanks, Yes Jim Comstock did the head. The Carbs were blown out and they do have the correct jets, i went over it with Andover before installing. Main jets are 230s not 220s
    Last edited: Sep 28, 2019
  5. MichaelB

    MichaelB VIP MEMBER

    Jan 14, 2004
    Has it been properly timed?
    Confirm plug wires have good connections. (I just went through this, had loose plug cap)
    Are chokes fully retracted?
    Work mixture screws 1/2 turn at a time.
    Good luck.
  6. dguzzi


    Jan 28, 2017
    I have noticed similar problems when the balance tube between the inlet manifolds has been eliminated. Really it needs to be there.
    comnoz likes this.
  7. brian4.2

    brian4.2 VIP MEMBER

    Jun 1, 2016
    Thanks. Chokes are fully retracted, new plug leads, all the coil spade connectors have been replaced. Timing is set to 31 degrees, static and @ 5000RPM. Boyer EI. Battery is new. Balance tube is there.

    I am thinking I could remove the chokes, I don't need them. But the slides are working fine. Throttle cable is free and moving the handle bars has no effect on RPM.
    Last edited: Sep 28, 2019
  8. Niagara850


    Jul 7, 2010
    Also running the DGR tomorrow. Does it do it when warm or just when cold? Mine won't idle cold and have to hold it. After a mile or so it idles at 1200-1500 (Amal Premiers). After 5 miles or so it settles down to 900. The DGR is a sloooow run but you should be able to blip your way thru it.
  9. brian4.2

    brian4.2 VIP MEMBER

    Jun 1, 2016
    All my adjustments are done when it is warmed up. It does pop a little when cold. I could move an idle adjustment 1/8 of a turn and gain 1000 RPM. It surges and drops.
  10. Niagara850


    Jul 7, 2010
    Have you adjusted the Pilot jet air screws? 1.5 out to start. Adjust till idle hits its highest point and screw in a tad (richen). Could also be an air leak. I know its all cleaned up but try spraying some WD 40 around the junctions; carbs, manifold and head. If the idles changes you've found it. Just don't over tighten the manifold to carb bolts...
  11. htown16


    Apr 29, 2009
    Sounds like it might be starved for fuel at idle. On the Premiers with the stayup floats don't set them at .080. What you really need to do is make a utube with an old drain plug and measure the fuel level with the taps on. Fuel level(not float level) should be at .21" +/- .005 below the top of the bowl. Bend the tab gently to get the level right. I've found that with the stayups with the fuel level set correctly, the top of the float is level and just slightly proud of the bowl with the bowl held level. http://amalcarb.co.uk/optimising-mark-1-concentric-fuel-levels
  12. Danno


    Feb 7, 2010
    Sometimes a fresh motor can be a bit tight for a good, sub-1000 rpm idle. Also, if you don't need chokes to start, you're getting too much fuel. What do the plugs show?
  13. o0norton0o


    Apr 27, 2015
    Ask yourself this, What on your engine has an "on, then off" pattern similar that would match the rhythm of the surging??... After all, the bike works, then it doesn't for a few seconds, then repeats... So, what is, "on/off/repeat" on a bike...

    ignition- NO, it usually works or doesn't work, it certainly wouldn't cycle like that.

    some mechanical part- NO, How would that even happen, where it fails for a few seconds then works for a few seconds? unlikely

    Fuel delivery- BINGO

    ....The most likely suspect is float related because the float turns the fuel "on and off" constantly as the bowl's drain then the flow cuts off so they don't overfill...

    ... Some fuel blockage at the petcocks or banjos could slow the flow down that the surge is fuel starvation. (If you own a chainsaw, the best way to keep the carbs in them clean is to drain the fuel, then run it out of gas at full throttle. When they are almost out of fuel the "lean out" and the engine surges before it dies when there's no fuel left) So we know that leaning out causes a surge *possible cause of your issue*

    ...Then there's a cause that is commonly suggested, that I've never seen in my 40 years of norton ownership. A blocked fuel cap breather causing a vacuum on the fuel delivery. possible, but rare.

    ... Last on this list is a cracked manifold or an air intake leak of some kind. There's no doubt that an air leak can cause surging, like I stated above in the chainsaw example but it wouldn't cycle "on/off/repeat". In my own bike's case, I had an invisible crack in the intake manifold that would only suck air when the bike heated up, but it didn't cause that cyclical surging that you are getting. I found it by spraying carb cleaner on the outside of the intake while the bike was running. When I sprayed the crack, which I could not even see, the bike would stumble. when I took the intake apart it fell into 2 pieces... and I found my invisible issue. Air leaks can also be at the adjustment screw rubber O-rings

    Good luck, do stuff, report back...

    *edited to add:

    The surge you are getting is your two cylinders fighting each other. One cylinder's carb needle is seated correctly in the inner carb body and the other cylinder's carb needle is wound around the spring so it's not all the way down. With the bike running at idle, it doesn't surge because the vacuum isn't pulling on the needle jet yet. Once you lift the slides a bit, you have one needle that's too high because it's not seated correctly and the bike is feeding different mixtures to the 2 cylinders... just another thing to check....
    Last edited: Sep 29, 2019
  14. Tornado

    Tornado VIP MEMBER

    Dec 5, 2017
    Needles at top notch? Why? Thought middle notch was recommended as initial setup.

    Check both needles properly clipped and seated at bottom of slides. Check they are both moving when you lift slides by feeling with fingers in the carb throats as throttle rolled.

    Recheck main and needle jets fully torqued. I had a main jet come undone after a few hundred miles. Gave a progressively worse misfire around 4k rpm as it continued ro unwind itself.

    Clean the idle jets. With premiers, not sure the old poke with a wire method still applies as the jets can be removed.

    Have you rechecked head bolt torques and then redone valve tappet clearances? Have heard newly fitted heads need retorquing several times within first number of miles.
  15. p400

    p400 VIP MEMBER

    Jul 14, 2014
    I had similar issues recently with a new pair of Premiers on my Mk3, wandering idle, slow to find idle, etc.
    240, 106, 19, mid notch, Norton needle, 3.5 slide, screws 1½, blah blah

    reset idle screws to 1¼ and all cleared up.

    Your needle position should not affect your idle
  16. comnoz

    comnoz VIP MEMBER

    Dec 10, 2008
    Have you checked the timing with a strobe. As blue as the pipes are I would suspect the timing is retarded. Jim
  17. htown16


    Apr 29, 2009
    Definitely strobe the timing. Lean in the mid-range due to having needles in the top groove could also blue the pipes. I'd put in the middle groove if I had the carbs apart. With that said, needle clip position shouldn't affect idle. What idle jets are you using? If they are 17's, you may need to go to 19's. Once the rebuild is complete is when the real work begins, sorting out all the issues and getting the bike to run right. Generally I've found it takes maybe a month or two. Take your time. Make one change at a time then road test. In the end you will have a bike you can enjoy.
    Gadge likes this.
  18. RoadScholar

    RoadScholar VIP MEMBER

    Dec 28, 2008
    Cam timing? 10 rollers between marks?

    New engines, in my experience, are hard to tune until the rings seat and the valves establish a seal.

    If the points made by previous members check with your empirical data pack a small plain blade screw driver where it can't be driven into you and go ride. Tweak the carbs as often you wish, they will eventually catch up with the the rest of the parts as they join the team.

  19. Joe Schlaberdowski

    Joe Schlaberdowski

    Jan 1, 2017
    Don't want to veer off one bike to another (mine) so if a new carb tuning thread 'll go ahead and get to that. Otherwise my issue with my 74 stock (I think) 850 is much the same but with VM34 Mikuni with a 30 pilot jet.
  20. brian4.2

    brian4.2 VIP MEMBER

    Jun 1, 2016
    Thanks for all the suggestions. This bike has only been ridden about 3 miles.

    Ok a little work done. The strobe timing is checked, 31 degrees, it tops out just above 4K RPM. It drops to 20 degrees at around 2K RPM. .Doesnt seem to get much lower

    I am working on the float heights. Stay up floats, viton tips in new 2 Amal Premiers. Still on the top groove. Pilot jets are clear. 23o mains. I do think it was running very lean.

    Todays Problem.

    After checking, the float heights were slightly over .080". I did set them to level with the top of the bowls, the gas ran out the ticklers freely, i don't think the Vtip ever closed the valve at this height. I couldn't start it like this. So I set them to .020" below , I think the valve never closes, gas is running out the ticklers again freely. Dont know what I am doing wrong. It is both sides. I have checked the metal frames for the float, seem straight and functioning properly.

    The ticklers dont seem to travel their full range, can you damage or change a float height by pushing too far?

    Sequence of events. Made the adjustment to .020" below the float bowl edge, put them back on the carbs. Opened the petcocks. No leaks. Used the ticklers and they never stopped running, both sides.

    Thanks Brian
    Last edited: Oct 6, 2019

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